Design controlled experiments to find out how a variable affects the quality of a product. Here are a few starters:

best size of tire for a race car

most-absorbent paper towel

best recipe for lemonade

most effective insulating material

best fabric for a raincoat

best way to heat water with solar energy

Double pendulums provide lots of interesting variables to investigate. Find out how changing the release heights, which pendulum is release, adding masses to one pendulum and not the other, or other possibilities affect the outcomes.

Make a double-decker pendulum by attaching a pendulum to the paper clip of another pendulum.

Hang two equal pendulums next to each other and link them with a soda straw that has been slop at each end.

Investigate stringless pendulums. Compare pendulums that are made from a variety of rigid materials, such as sticks, straws, paper clips, or wire. Compare these pendulums without adding masses such as pennies.

Does the kind of liquid a boat floats in have an effect on the number of passengers it can support? Investigate the effect of heavily salted water or any other safe liquid.

Conduct controlled experiments to investigate the variables that affect the use of any of the following toys: windup car, toy parachute, Frisbee, yo-yo, bicycle, stakeboard, paper airplane, cassette player, football, and others.

Make balloon rockets. Tape a soda straw to one edge of a plastic bag (a 1-liter zip bag is a good size) suspended from a flight line. Blow up a long balloon and put it into the bag wile holding the balloon shut. When you release the balloon, the rocket will shoot down the line. Conduct controlled experiments to investigate the variables that might affect the length of flight.

Investigate compensating variables in a flip-stick system. Set up a target, such as a cup, and launch a foil ball so that it hits the target. Then change one of the variables and hit the target again. In order to do so you will have to compensate for the changed variable by changing one or more other variables.

Make a coin sorter, using a flipper system. Position 1/2-liter containers at strategic locations so that, when any coin is flipped, it will land in the container with the other coins of its kind.